Introduction of volatile liquid additives to an ion mobility buffer gas is known in ion mobility spectrometry (“IMS”) devices that operate at atmospheric pressure. Liquid additives can be introduced as a vapour into the buffer gas stream, and the drift gas of the ion mobility separation device may comprise buffer gas as well as the additives dispersed therein. The volatility range and mol fraction of the additive as a proportion of the drift gas composition may be limited to avoid condensation of the additive within the ion mobility separation device. The ion mobility separation device may need to be heated above the boiling point of the liquid additive in order to avoid condensation.
If condensation of the liquid additive occurs within the ion mobility separation device, the mol fraction of the additive in the drift gas cannot be accurately calculated and may not be constant throughout all separations. Consequently it can be difficult to quantitatively introduce liquid additives into atmospheric pressure ion mobility separation devices.
An improved method of separating ions in which one or more substances, for example liquid additives, are dispersed or otherwise present in the drift gas of an ion mobility separation device is desired.